Platelet-derived growth factor (PDGF) is a polypeptide that can be produced by diverse cells and can stimulate the proliferation of stroma-derived cells. In the 1970s, it was first discovered by Ross et. al. from the platelet, and thus named (1). So far, a total of four PDGF monomers, namely, PDGF-A, PDGF-B, PDGF-C and PDGF-D, have been found. These monomers form five kinds of homo-or hetero-dimers with each other via intrachain and interchain disulfide bonds: PDGF-AA, PDGF-BB, PDGF-AB, PDGF-CC, and PDGF-DD (2, 3). It is generally accepted that PDGF genes and proteins belong to a family of structurally and functionally related growth factors, which also includes vascular endothelial growth factor (VEGFs) and placental growth factor (PIGF) (4). PDGF plays a physiological role by activating its receptor PDGF-Rs. PDGF-Rs include both PDGFR-α and PDGFR-β, belonging to tyrosine kinase receptor. Binding of a ligand to a receptor triggers the dimerization of receptor monomers, leading to the autophosphorylation and activation of intracellular tyrosine residues. Both receptors can activate the critical molecules in multiple signaling pathways, such as Ras-MAPK, PI3K and PLC-γ (5), in turn activate transcription of the related genes, stimulate cell growth, inhibit apoptosis, promote differentiation and induce oriented movement, migration, and the like, and play a variety of biological functions.
PDGF-b gene is located on chromosome 22 and contains seven exons. It encodes a precursor protein consisting of 241 amino acids and its final mature product formed by proteolytic processing is a polypeptide consisting of 109 amino acids, with a molecular weight of 12.3 kD. In organisms, the active form of PDGF-B protein is homodimer PDGF-BB or heterodimer PDGF-AB formed from two monomers via disulfide bonds (6). Each PDGF-B protein monomer contains eight highly conserved cysteine residues, wherein six cysteines form intrachain disulfide bonds (Cys I-VI, III-VII, V-VIII), and the other two form interchain disulfide bonds with the corresponding monomers (Cys II-IV) (7), together forming the growth factor domains-characteristic of the PDGF protein family, i.e., cystine knots. These intra-and inter-chain disulfide bonds constitute a complex spatial structure of the PDGF-BB dimer protein (FIG. 1B).
In addition, different splicing forms of PDGF protein exist during the expression and synthesis of the protein, so that the PDGF protein upon processing and maturation exhibits a variety of structural forms. N-terminal amino acid sequence analysis of the PDGF-BB isolated and purified from human platelet extracts indicates the presence of at least three different splicing forms, 20% Ser1, 45% Thr6 and 35% Thr33. The heterogeneity of these cleavage products renders the proportion of proteins in various cleavage forms uncontrollable when purifying PDGF-BB (8).